1. Field of Invention
The invention relates to spool valves having a ported valve chamber and a valve spool mounted for reciprocation therein and carrying an O-ring in sliding engagement with the chamber wall for controlling fluid flow through the valve port therein.
2. Description of Prior Art
Spool valves of the character described have used two basic types of O-rings. The usual commerical style O-ring is compressed radially so as to provide simultaneous sealing of the ring at its outside diameter and its inside diameter (with the root surface of the ring groove). When properly installed, this type of O-ring provides a very satisfactory seal. However, the requirements of commercial tolerances in the parts require designing for a cnsiderable amount of squeeze or compression with attendant high friction forces. Customarily, relatively large size O-rings, that is, having a relatively large torus section, are used to obtain a reduced ratio of squeeze to cross-sectional area and smaller radial compression forces. For example, in spool valves having O-rings up to approximately 1/2 inch OD, the commercially recommended diameter of the torus section is 0.070 inch; and the recommended torus section diameter is 0.103 inch for O-rings of 1/2 inch to 1 inch OD.
The use of a so-called "floating" O-ring has been proposed to overcome the aforementioned high frictional forces. In this type of seal ring, the compression of the O-ring against the adjacent cylindrical surface is maintained, but the internal diameter of the O-ring is not forced into contact with the root wall of the ring groove. Sealing is effected at the side walls of the groove and the adjacent cylindrical surface. Substantially less friction is normally found in this type of seal. However, to obtain a seal it is essential that the O-ring be moved laterally across the interior of the ring groove and into sealing engagement with one of the groove side walls. Such displacement will dependably occur when there is a rapid application of pressure sufficient to move the ring into its sealing position. When the pressure builds up slowly, however, as may occur in many applications, leakage will occur around the ring.
Another disadvantage found in the relative large size O-rings heretofore used is the attendant relatively large movement required of the spool valve in effecting its valving action and to provide adequate fluid flow passage. Accompanying such large required movement is also a lack of precision. Generally, the "cut-off" or sealing point is better defined when a narrow seal band width is used than when a wide band is used resulting from a highly compressed large section O-ring.